Polyester films are excellent in mechanical and chemical properties, and are used in a wide range of fields such as those of packaging, magnetic tapes, electronic components, electrical insulation and protective sheets. In solar cell back sheet applications, solar cell front sheet applications, motor insulation applications and capacitor applications, the films are exposed to a severe environment for a long period of time, and therefore a higher level of durability is required. However, when the polyester film is used in a high-temperature and high-humidity environment, there is a problem that hydrolysis occurs at the ester bond site in the molecular chain, so that mechanical properties become worse. Thus, various studies have been conducted for improvement in hydrolysis-resistant performance of polyester films.
For example, Patent Document 1 discloses a technique of improving hydrolysis resistance by lowering the acid value of a resin. Patent Documents 2 to 4 disclose techniques of improving weather resistance by increasing the molecular weight of a resin.
Patent Documents 5 to 7 disclose techniques of adding a carboxylic acid end capping agent such as an epoxy compound, an oxazoline compound or an isocyanate compound for improving the hydrolysis resistance of a polyester film. A hindered phenol-based antioxidant is used in parallel therewith as an additive for prevention of deactivation of an end capping agent and prevention of gelation. Further, Patent Documents 8 to 10 disclose techniques of improving weather resistance and various properties by homogeneously mixing and blending a polyester and other resins. A hindered phenol compound is also used in parallel therewith as an additive for preventing decomposition and coloring of a resin to be blended.
As to the hindered phenol-based compound, a technique of preventing heat aging of a molded product by adding the compound as an antioxidant has been previously widely known in the art of engineering plastics such as injection molding (Patent Document 11).